On my system (X1900XT CF) Rome Total War works okay, so it probably isn't a
driver bug.
Few things to try:
1. Set Catalyst AI to maximum in CCC
2. If the game was installed before you added both video cards, reinstall
the game so it can redetect the hardware config.
3. Disable Crossfire in CCC. I suspect the game is very much CPU-limited so
the second card doesn't improve anything.